This invention relates to a surface position detecting method, a surface position adjusting apparatus, and a projection exposure apparatus. More particularly, the invention relates to a surface position detecting method for detecting a surface position in which a pattern image of a first object is formed through a projection optical system within a projection area on the surface of a second object, a surface position adjusting apparatus for bringing an area of the surface of the second object into alignment with the image plane of the projection optical system, and a projection exposure apparatus using the surface adjusting apparatus.
Various types of exposure apparatus are known that are used in a photolithographic process for manufacturing a semiconductor device, liquid crystal display or others. Typically, an exposure apparatus is used that exposes and transfers the pattern of a photomask or reticle (comprehensively referred to as a reticle) through a projection optical system onto a wafer or substrate coated with a photosensitizer.
For example, a reduction projection exposure apparatus for manufacturing a semiconductor device uses a projection lens system having a large numerical aperture (N.A.). In such an exposure apparatus, the focal depth is very shallow, and therefore, a mechanism is provided for bringing the wafer surface into alignment with the image plane of the projection lens system on the substrate. A conventional reduction projection exposure apparatus has an autofocus mechanism for adjusting the vertical position of the wafer surface along the optical axis of the projection lens system and a leveling mechanism for adjusting the inclination of the wafer to make the wafer surface align with the image plane of the projection lens system on the substrate. Japanese Patent Application No. 5-190423 (U.S. Pat. No. 5,502,311), discloses an oblique incidence type exposure apparatus having a multipoint AF system, in which the vertical position along the optical axis and inclination of the wafer are simultaneously detected. With the multipoint AF system, the vertical positions, along the optical axis of the projection lens system, of three or more points on the wafer surface are detected to calculate the inclination of the wafer surface.
The conventional autofocus mechanism, however, generally detects the vertical position of the substrate surface only in a portion of the exposure area, even with a multipoint AF system. If there are level differences in an exposure area to apply, for example, one-shot-four-chip or one-shot-six-chip exposure, these level differences cannot be detected accurately due to the limited number of detection points. Consequently, the focussing operation of the substrate can be inaccurate.